Field of the Invention
The present invention relates to a field of industrial equipment robots, and in particular, to a servo press capable of preventing a step-out phenomenon from occurring and of realizing an improvement of takt time, as well as to a control method and a recording medium recording a program.
Description of Related Art
As a motor for driving slide blocks and robots for carrying workpieces, industrial machines such as machine tools, office automation equipment, and terminal equipment and peripheral equipment of a computer, AC and DC servomotors and pulse motors have been widely employed.
Among these, servomotors are able to move driven members such as slide blocks to a predetermined position at a predetermined speed by feedback control based on a closed loop, and therefore widely employed in driving systems of machines, such as machine tools, that require accurate positioning. On the other hand, such a servomotor, along with its peripheral equipment, tends to be increase the cost of a driving system. In contrast, with pulse motors, it is possible to perform open loop control based on signals input from outside, and to construct a driving system at a relatively low cost. Therefore their use in office automation equipment and the like have become increased.
In the meantime, a pulse motor generates torque by delaying a rotor with respect to a rotating magnetic field due to an excitation current. A delay angle of the rotor to the rotating magnetic field is referred to as a load angle, and as an inertia force and a frictional force increases, the load angle also increases. Here, if the load angle exceeds an angle when the generated torque is maximized, a total of the inertia force and the frictional force exceeds maximum torque of the motor, and the rotor may not follow the rotating magnetic field. In a case of such radical speed change or overloading, the rotor may not rotate following a command pulse, the pulse motor stops, or positional deviation occurs. This phenomenon is called step-out.
In view of such circumstances, there is disclosed a technique of alarming occurrence of step-out, in which an excitation step counter for controlling an excitation step angle based on an input pulse train signal and counting a number of excitation steps, and an encoder counter for counting output signals from a rotary encoder for detecting a rotational angle of a pulse motor and outputting an encoder counter value indicating an actual rotational angle of the pulse motor are disposed, and angular deviation of the pulse motor is calculated by comparing the number of excitation steps with the encoder counter value to determine whether or not there is step-out (e.g., PTL 1).